ADE7761A
Looking at Table 10, the closest frequency to 0.266 Hz
in Column 4 is 0.17 Hz. Therefore, F2 (3.4 Hz; see Table 6) is
selected for this design.
For example, an energy meter with a meter constant of
100 impulses per kWh on F1, F2 using SCF = 1, S1 = 0, and
S0 = 1, the maximum output frequency at F1 or F2 is 0.68 Hz
and 43.52 Hz on CF. The minimum output frequency at F1
or F2 is 0.0045% of 0.68 Hz or 3.06 × 10–5 Hz. This is 1.96 ×
10–3 Hz at CF (64 × F1 Hz).
Frequency Outputs
Figure 2 is a timing diagram for the various frequency outputs.
The high frequency CF output is intended for communication
and calibration purposes. CF produces a 90 ms wide, active
high pulse (t4) at a frequency that is proportional to active
power. The CF output frequencies are given in Table 8. As with
F1 and F2, if the period of CF (t5) falls below 180 ms, the CF
pulse width is set to half the period. For example, if the CF
frequency is 20 Hz, the CF pulse width is 25 ms.
In this example, the no-load threshold is equivalent to 1.1 W of
load or a startup current of 4.6 mA at 240 V. Compare this value
to the IEC 32053-21 specification, which states that the meter
must start up with a load equal to or less than 0.4% of IB. For a
5 A (IB) meter, 0.4% of IB is equivalent to 20 mA.
Note that the no-load threshold is not enabled when using the
high CF frequency mode: SCF = 0, S1 = S0 = 1.
No-Load Threshold
The ADE7761A includes a no-load threshold and start-up
current feature that eliminates creep effects in the meter. The
ADE7761A is designed to issue a minimum output frequency.
Any load generating a frequency lower than this minimum
frequency does not cause a pulse to be issued on F1, F2, or CF.
The minimum output frequency is given as 0.0045% of the full-
scale output frequency. (See Table 8 for maximum output
frequencies for ac signals).
NEGATIVE POWER INFORMATION
The ADE7761A detects when the current and voltage channels
have a phase shift greater than 90°. This mechanism can detect
a wrong connection of the meter or the generation of negative
power. The REVP pin output goes active high when negative
power is detected and active low when positive power is
detected. The REVP pin output changes state as a pulse is
issued on CF.
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